The flyback converter is an isolated power converter that is commonly used in both AC-to-DC and DC-to-DC conversion with galvanic isolation between the input and the one or more outputs. More specifically, the flyback converter is a buck-boost converter with the inductor split to form a transformer, so that the voltage ratios are multiplied with an additional advantage of isolation. Synchronous rectification is often applied to replace the diode rectifier to improve efficiency. FIG. 1 is an example of a flyback converter using synchronous rectification. As shown in FIG. 1, a typical construction of the flyback converter includes a primary switch (SW) coupled to the primary transformer winding of a transformer Lm and a synchronous rectifier switch (SR) coupled to the secondary transformer winding of the transformer Lm. The input voltage VIN is provided across the primary winding and the primary switch. The primary switch is controlled by a control voltage VGS to turn on and off to conduct a primary current Ipri. The primary switch and the synchronous rectifier are complementary in operation with one switch being turned on while the other switch is turned off. The conduction periods of the primary switch SW and the synchronous rectifier SR do not overlap. The current flowing on the secondary side, referred to as the secondary current Isec, charges an output capacitor C3 to provide the output voltage Vo. In some cases, active clamping may be implemented at the primary side to clamp the voltage at the drain terminal of the primary switch SW when the primary switch SW is turned off.
FIG. 2 illustrates exemplary signals waveforms for operating the flyback converter of FIG. 1 in a constant frequency, continuous-conduction-mode (CF CCM). FIG. 3 illustrates exemplary signals waveforms for operating the flyback converter of FIG. 1 in a constant frequency, discontinuous-conduction-mode (CF DCM). The flyback converter of FIG. 1 and the operation modes of FIGS. 2 and 3 are described in detail in the paper by M. T. Zhang, M. M. Jovanovic and F. C. Lee, “Design considerations and performance evaluations of synchronous rectification in flyback converters,” Applied Power Electronics Conference and Exposition, 1997, APEC '97 Conference Proceedings 1997, pp. 623-630 vol. 2. In short, when operated in the CCM operation mode, the secondary current Isec does not go to zero current value before the start of the next switching cycle (primary switch SW turns on), as shown in FIG. 2. On the other hand, when operated in the DCM operation mode, the secondary current Isec go to zero current value before the start of the next switching cycle, as shown in FIG. 3.
Flyback converters suffer from power losses when switching transitions occur with non-zero voltages across the power switches. Zero voltage switching (ZVS) has been implemented in flyback converters to accomplish switching at zero voltage so as to achieve high efficiencies. Various techniques for implementing zero voltage switching have been described. For example, the Zhang paper describes a variable frequency (VF) ZVS DCM operation mode that can be implemented in the flyback converter of FIG. 1. FIG. 4 duplicates FIG. 5 of the Zhang paper and illustrates exemplary signals waveforms for operating the flyback converter of FIG. 1 in a VF ZVS DCM operation mode. In particular, to achieve ZVS at the primary switch, the conduction period of the synchronous rectifier (SR) is extended, or the turn off time of the synchronous rectifier is delayed by a time period Tdelay after the secondary current Isec has already reached zero current value. The extended on time Tdelay is used to allow a negative secondary current to build up on the secondary transformer winding, as indicated by current IZVS in FIG. 4. As long as the energy stored in the magnetizing inductance Lm by the negative secondary current IZVS is sufficient to discharge the primary switch parasitic capacitance (denoted by C1 in FIG. 1) down to zero voltage, ZVS can be achieved in the flyback converter. However, with the use of the extended SR conduction period, the switching frequency of the flyback converter becomes variable as a function of the load regulation. Variable frequency operation is undesirable, especially when avoidance of electromagnetic interference (EMI) disturbance is important.